Generally cementing a well consists of pumping cement slurry from the surface down the casing so that it then returns to the surface via the annulus between the casing and the borehole. One of the purposes of cementing a well is to isolate the different formation layers traversed by the well to prevent fluid migration between the different geological layers or between the layers and the surface. For safety reason it is also essential to prevent any gas rising through the annulus between the borehole wall and the casing.
When the cement has set, it is impermeable to gas. Because of the hydraulic pressure of the height of the cement column, the injected slurry is also capable of preventing such migration. However, there is a critical phase, between these two states which lasts several hours during which the cement slurry no longer behaves as a liquid but also does not yet behave as an impermeable solid. For this reason the industry has developed a series of additives which are intended to maintain a gas-tight seal during the whole cement setting period.
Fluid loss is an important property to control in cement slurries. Fluid loss occurs when the cement slurry comes into contact with a highly porous or fissured formation. Fluid from the cement slurry will migrate into the formation altering the properties of the slurry. When fluid loss occurs it makes the cement more permeable to gas. Fluid loss control additives are used to prevent or at least limit the fluid loss that may be sustained by the cement slurry during placement and its setting.
Known additives include liquid additives based either on a styrene-butadiene latex or a cross-linked poly(vinyl)alcohol microgel.
U.S. Pat. No. 4,537,918 describes using a liquid styrene/butadiene latex as a gas migration prevention agent. The liquid is added to the cement slurry to control gas channeling in the cement.
U.S. Pat. No. 6,235,809 describes a polymer-based additive for gas migration prevention. The additive comprises a blend of sodium silicate, water, a carrageenan suspending agent, sodium polyacrylate, an AM/AMPS/N-vinyl-N-methyl acetamide terpolymer, a dispersant and a cellulosic water soluble polymer. The additive can be either in a dry or aqueous form. The additive is added to the cement slurry to reduce the occurrence of gas channeling in the cement in light-weight-low density cement compositions. However AMPS based copolymer additives are expensive.
In addition, in locations where the climate is cold, such as Russia, Alaska, and Canada for example, liquid additives are not appropriate. In cold climates the liquid additives are difficult to handle as they become hard and therefore are not as pourable, which can lead to difficulties in proper mixing in the cement slurry.
Therefore the invention proposes a solid gas migration prevention additive comprising a latex powder for cementing wells which can be blended with the dry constituents of the slurry. The additive can be used in low temperature climates.